Induction motor rotor



April 2, 1940. y w, R COX 2,186,059

l INDUCTION MOTOR ROTOR Filed Feb. 25, 1938 Inventor: William R. Cox,

b g x/yAttorneg.

Patented Apr. 2, 1940 UNITED STATES,

PATENT OFFICE INDUGTION MOTOR BOTOB Application February 25,1938, SerialNo. 192,606

Claims.

My invention relates to the secondary rotor constructions of squirrelcage induction motors and its objects are to provide a rotor which iseasyto manufacture by the casting method, is strong in constructionsothat it willwithstand high speed and reversing stresses, has highresistance and has an. Yend construction in the form of a fan to keep itfrom excessive heating during operation. 'I/'he construction also lendsitself to a winding in which the reactance may be varied considerablyyby the optional use of magnetic material associated with one or bothendsof the winding. v

The casting of high resistance squirrel cage windings for medium sizedmotors has heretofore not been universally successful or practicable.Successful casting necessitates that the bar slots shall have suilicientcross-section to cause the molten casting metal to flow freely thereinduring the casting operation and also it is desirable that the barsshall be of suflcient crosssection and have such mechanical connectionwith the end rings as to withstand high temperatures and mechanicalstresses. These factors call for a relatively large bar cross-sectionwhich is inconsistent with high resistance unless special and expensivehigh resistance alloy material is employed.

According to my invention, a slot and bar crosssection consistent withsuccessful casting and mechanical strength is used and the requisitehigh resistance is obtained by extending the length of the secondarycurrent path without `greatly extending the length of the rotor. This isaccomplished by providing one or more loops in the bars between therotor core and end ring, preferably at both ends of the rotor. shape ofthe bars is easily obtained by using suitable molds and casting thewinding, and the resulting structure is mechanically strong and of suchshape that the loops act as effective cooling fan blades when the rotoris running.

The features of my invention which .are be- -lieved to be novel andpatentable will be pointed out in the claims appended hereto. For abetter understanding of my invention reference is made in the followingdescription to the accompanying drawing in which Fig. 1 represents acomplete induction motor to which my invention pertains. Fig. 2 is aperspective view partially in section of an embodiment of my inventionwhich may include magnetic ring material between the sides of the loopsin the squirrel cage bars for increasing the reactance of the wind- I6ing; Fig. 3 is another modication of the invention with no magneticmaterial within the loops or bends of the squirrel cage bars and Figs. 4and 5 represent other forms of loops or grids that may be used. y

In Fig. 1, I0 represents the stator ofl an induction motor and II therotor assumed to have been constructed in accordance with the presentinvention. The part marked I2 may represent lthestator winding endshields or the end turns of the stator windings themselves. It will benoted that the rotor construction does not add appreciably to the axiallength of the motor shown. The motor bearings are not shown but it willbe evident that the slightly increased length of the rotor will notinterferewith the usual u type of bearings as substantially normallyspaced at the two ends of the rotor and hence no material over-al1increase in motor length is required by reason of the present invention.

In Fig. 2, I3 represents the usual rotor core 20 made up of laminatedmagnetic material having slots I4 in its periphery and shaft opening 20.'I'he slots and the bars I5 therein are appreciably relatively larger ascompared to the rotor diameter than would be called for in a rotor of 5'usual design and having an equivalent high `resistance squirrel cagewinding. The reason for the larger slots is that if the rotor is to becast and it is to be mechanically strong and able to withstand hightemperatures, the slotsl should be large enough to permit them to becompletely filled with the molten metal without leaving cavities and thecross-section of the bars should be suflicient to have appreciable heatstorage capacity below a melting temperature and to u withstandmechanical stresses incident to quick starting and stopping and highspeed operation. Generally speaking, heretofore in rotors of relativelysmall sizes such as are generally used in motors below 15 horse-power,the bar size cono sistent with a high resistance rotor was too small forthe squirrel cage to be cast in a satisfactory manner. Increasing thesize of the bars for mechanical reasons obviously lowers the resistanceof the squirrel cage. According to my 5 invention the bars are madelarge enoughugp be cast in a satisfactory manner and to be sufficientlystrong mechanically and the reduction in resistance per unit of lengthis made up by an increase in the eective length of secondary currentpath by the loop-shaped extensions at the ends Vof the bars. Thus, inFig. 1 the bars I5 have an inwardly extending radial connecting loop I6at each end before they are joined by the end rings I1. The straight andlooped bar sections and the end rings are, or may be, cast in oneoperation by providing suitable molds, which molds preferably includethe ring punchings shown at I8 and I9.

Where it is Adesirable to increase the rotor reactance or to give it theequivalent of some double squirrel cage eilect, one or both of the ringsI8 and I9 at the ends of the rotor may be made of laminated magneticmaterial, such as is used in the rotor laminations I3, and left in placeafter the squirrel cage is cast. Likewise, one or both rings at eitherend may be made in sections so as to be removed after the winding iscast. Where such rings are left in the rotor for increasing thereactance or for mechanical strength, they will beV made of a highresistance material or have an insulating coating or scale so as not toshort circuit the squirrel cage bars. It would also be possible to castthe squirrel cage material without using the' molding rings I8 and I9,and after the casting operation, to place the rotor in a lathe and cutgrooves in the places which are shown asbeing occupied by such rings.

It is seen that the structure represented considerably increases thelength to the path of the current in a squirrel cage bar by reason ofthe added radial directions in which such current must ilow in passingbetween the end rings II at opposite ends of the rotor but withoutcorrespondingly increasing the length of the rotor.

It is seen that the loops in the various bars form with the barsa seriesof eifective clamps about the rotor for the core laminations I3 and thestructure is otherwise mechanically well anchored-strong and rugged. Theradial loops or grids make eiective centrifugal Ventilating fans atopposite ends of the rotor which assist in keeping the rotor cool. Itwill be noted that these fans are very effectively placed at the pointswhere the rotor winding resistance is most highly concentrated and wheremaximum heating is likely to occur and afford a large heat radiatingsurface directly in the path of the fan blast.

In Fig. 3 a modiiled form of the invention is shown at one end of therotor only. In this case the loops 2I are formed of U-shaped, inwardlyturned loop or grid sections which are joined by an end ring 22 at thesmaller diameter next to the rotor core. Here again the extra grid-likeresistance material is confined within the diametrical limits of therotor core and utilizes space at the ends of the rotor radially inwardfrom the rotor periphery. It provides thefan construction, added windingresistance and is mechanically satisfactory in strength.

In this case the open end of the U-shaped loops face axially inward withthe outer limbs ofthe loops comprising axial extensions of the bars. Theend ringsare secured against the ends of the laminated core lbv theirconnections with the inner legs ofthe loop.

Other examples are represented in Figs. 4 and 5 where I3 represents therotor core', I5 the squirrel cage bar and 22 the end ring. Each squirrelcage bar is extended to the end ring by a resistance grid loop sectionwhich also forms a radial fan blade. The length, size and number ofresistance loops or grid sections may be varied to suit the designrequired. So far as possible, however, the loop end sections` willconform to the shape and disposition'of radial fan blarl and theseconnections as well as the end rings will be confined within theperipheral dimensions of the rotor so as not to increase its maximumdiameter( The material used in the squirrel cage winding may be anysuitable conductor material that can be cast, for example, aluminum orcopper.

What I. claim as new and desire to secure by Letters Patent of theUnited States is:

l. An induction motor secondary rotor comprising a laminated magneticcore containing peripheral slots and a high resistance squirrel cagewinding having bars contained in said slots, end rings at the ends ofthe rotor and looped connections between said bars and at least one endring for increasing the resistance of the winding, said loopedconnections forming radial fan blades at the end of the rotor, saidsquirrel cage winding comprising a casting and the end portions thereofforming clamping structure for the core laminations.

2. An induction motor secondary rotor comprising a magnetic corecontaining peripheral slots, a high resistance, cast squirrel cagewinding having bars in said slots, end rings therefor and loopedshaped'connections between said bars and end rings extending beyond theends oi the core and forming fans at either end of the rotor and a ringof magnetic material secured in the slots of such loops at least at oneend of the rotor for increasing the reactance of said winding.

3. A secondary rotor for induction motors comprising a laminated coreoi' magnetic material containing peripheral slots, a high resistancesquirrel cage winding having bars cast in said slots, end rings thereforand connections between said bars and end rings cast with said bars,said connections comprising radially extending loops confined within theouter periphery of the core and occupying space at the ends of the core,the loops at either end of the rotor forming a ian of which each loop isa radial blade, said loops and bars also serving as clamps for thelaminated rotor core.

4. A secondary rotor for induction motors comprising a laminatedmagnetic core having slots in its periphery, a high resistance, castsquirrel cage winding having bars in said slots, end rings therefor andU-shaped loop connections between the ends of the bars and the endrings, said U- shaped connections extending radially inward from thebars and end rings and snugly fitting over the end laminations of therotor core with the opening in the loops facing outward, said loopsAforming centrifugal Ventilating fans at either end ci the rotor.

5. A secondary rotor for induction motors com'- prising a laminatedmagnetic core having peripheral slots, a cast squirrel cage windinghaving bars in said slots, said bars having grid-like extensionsextending beyond the ends of the core and inwardly within the peripherallimits thereof for the purpose of increasing the length of the rsquirrel cage current path and its resistance without correspondinglyincreasing the length of the rotor, and end rings havinga maximumdiameter no greater than that of the core joining the otherwise freeends of said extensions, said extensions being disposed to. formcentrifugal fans at the ends of thewrotor and also serving as clampingstructure at the ends of the rotor core Lo prevent axial displacementoi' the core laminaons. WILLIAM R. COX.

